ESA Science & Technology - Publication Archive

The Sun from Space is a comprehensive account of solar astrophysics and how our perception and knowledge of this star have gradually evolved as mankind has elucidated ever more of its mysteries. The emphasis is on the last decade, which has seen three successful solar spacecraft missions: SOHO, Ulysses and Yohkoh. Together these have confirmed many aspects of the SUN and its output, and provided new clues to the numerous open questions that remain. The author, a leading researcher in the field, writes in a clear and concise style. Known also for his famous books "Astrophysical Formulae", "Sun, Earth and Sky", and the prize-winning "Wanderers in Space", he has succeeded once again in addressing a complex scientific topic in a very approachable way. Hence, this generously illustrated book, whilst primarily addressing students, will also be of interest to a broader readership covering all levels from the amateur to the expert.

Workshop held 22-25 June 1999 in Paris, France.

Proc. Kanzelhoehe Summer School, eds. A. Hanslmeier and M. Messerotti, Kluwer, 2001

Kosovichev, A.G., Duvall, T.L. (eds.)

SUMER - the Solar Ultraviolet Measurements of the Emitted Radiation instrument on the Solar and Heliospheric Observatory (SOHO) - observed its first light on January 24, 1996, and subsequently obtained a detailed spectrum with detector B in the wavelength range from 660 to 1490 Å (in first order) inside and above the limb in the north polar coronal hole. Using detector A of the instrument, this range was later extended to 1610 Å. The second-order spectra of detectors A and B cover 330 to 805 Å and are superimposed on the first-order spectra. Many more features and areas of the Sun and their spectra have been observed since, including coronal holes, polar plumes and active regions. The atoms and ions emitting this radiation exist at temperatures below 2 ×106 K and are thus ideally suited to investigate the solar transition region where the temperature increases from chromospheric to coronal values. SUMER can also be operated in a manner such that it makes images or spectroheliograms of different sizes in selected spectral lines. A detailed line profile with spectral resolution elements between 22 and 45 mÅ is produced for each line at each spatial location along the slit. From the line width, intensity and wavelength position we are able to deduce temperature, density, and velocity of the emitting atoms and ions for each emission line and spatial element in the spectroheliogram. Because of the high spectral resolution and low noise of SUMER, we have been able to detect faint lines not previously observed and, in addition, to determine their spectral profiles. SUMER has already recorded over 2000 extreme ultraviolet emission lines and many identifications have been made on the disk and in the corona.

SOHO, launched by an Atlas II-AS from Cape Canaveral on 2 December 1995, was inserted into its halo orbit around the L1 Lagrangian point on 14 February, six weeks ahead of schedule. The launch and the orbital manoeuvres were so accurate and efficient that sufficient fuel remains on board to maintain the halo orbit for more than a decade, i.e. for at least twice as long as originally foreseen. Already during their commissioning phase, the SOHO experiments have returned a wealth of data, impressive in terms of both its quality and diversity. Some of the images can be viewed via the SOHO pages (http://sohowww.nascom.nasa.gov) on the World Wide Web, and on the individual experiment pages, all with links from the SOHO home page. Typical examples of the unique results being obtained with SOHO's instruments are presented here. Although they have been obtained with single instruments, it is worth noting that the main scientific advances from SOHO are expected to come from the joint analysis of coordinated observations.

A couple years ago researchers announced that they had discovered traces of water on the sun. A team at Stanford University has just gone them one better, discovering entire rivers on the sun. These are not rivers in the familiar sense, of course; rather they are huge, snaking flows within the white-hot plasma (electrically charged gas) that makes up the sun.

Images of an active region on the far side of the Sun were derived by applying seismic holography to recent helioseismic observations from space. Active regions are the centers of energetic phenomena such as solar flares and coronal mass ejections whose resulting electromagnetic and particle radiation interfere with telecommunications and power transmissions on Earth and can pose significant hazards to astronauts and spacecraft. Synoptic seismic imaging of far-side solar activity will now allow us to anticipate the appearance of large active regions more than a week ahead of their arrival on the east solar limb.

Observations of outflow velocities in coronal holes (regions of open coronal magnetic field) have recently been obtained with the Solar and Heliospheric Observatory (SOHO) spacecraft. Velocity maps of Ne⁷⁺ from its bright resonance line at 770 angstroms, formed at the base of the corona, show a relationship between outflow velocity and chromospheric magnetic network structure, suggesting that the solar wind is rooted at its base to this structure, emanating from localized regions along boundaries and boundary intersections of magnetic network cells. This apparent relation to the chromospheric magnetic network and the relatively large outflow velocity signatures will improve understanding of the complex structure and dynamics at the base of the corona and the source region of the solar wind.

Solar flares involve a release of the Suns magnetic energy as X-radiation, particle beams and high-speed plasma flows. But we have discovered, using data from the Solar and Heliospheric Observatory (SOHO), that these flares also affect the Suns interior, generating seismic waves similar to earthquakes. For example, a three-kilometre-high seismic wave was caused by a moderate X-ray flare that occurred on 9 July 1996 and propagated at about 50 kilometres per second to a distance 120 000 kilometres from the flare site.